1. Field of the Invention
The present invention relates to a honeycomb catalyst body for purification of an exhaust gas.
2. Description of Related Art
In an exhaust gas discharged from an internal combustion engine such as a car engine, harmful substances such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) are included. When these harmful substances are decreased to purify the exhaust gas, a catalytic reaction has broadly been used. In this catalytic reaction, a harmful substance such as carbon monoxide (CO) can be converted to a harmless substance by a simple method of bringing the exhaust gas into contact with a catalyst. Therefore, in the car or the like, it has been usual that the catalyst is disposed in the middle of an exhaust system of the exhaust gas, to perform the purification of the exhaust gas.
For the purification of the exhaust gas by use of the catalyst, a honeycomb catalyst body has broadly been employed in which the catalyst is loaded onto a honeycomb structure. In the honeycomb catalyst body, the honeycomb structure is formed by partition walls onto which the catalyst is loaded. When the catalyst is loaded onto the partition walls of the honeycomb structure in this manner, a surface area of the catalyst per volume of the catalyst body is large. Therefore, the exhaust gas comes in contact with the catalyst at high frequency. Consequently, in the honeycomb catalyst body, the catalytic reaction is promoted to enable efficient purification of the exhaust gas.
As the above-mentioned catalyst to be loaded onto the honeycomb catalyst body, a zeolite is used sometimes. A metal-substituted zeolite (e.g., a copper ion-exchanged zeolite or an iron ion-exchanged zeolite) is used as an SCR catalyst for NOx selective reduction in the honeycomb catalyst body for the purification of the exhaust gas (e.g., Patent Document 1). Moreover, the zeolite has a function of adsorbing the HC and the like. When the zeolite is used together with a noble metal catalyst of Pt or the like in the honeycomb catalyst body, the zeolite can adsorb the HC and the like at a low temperature at which the noble metal catalyst is not activated, and can retain the HC and the like as they are until reaching a high temperature (at which the noble metal catalyst is activated). Therefore, it is possible to further increase a purification efficiency of the HC and the like (e.g., Patent Documents 2 and 3).
The catalyst function of the zeolite tends to enhance depending on an amount of the zeolite. Therefore, to intend the enhancement of the purification efficiency of NOx, there has been suggested a modification technology of increasing a cell density of the honeycomb catalyst body while increasing or maintaining the amount of the zeolite to be loaded onto the honeycomb catalyst body [the amount (g/L) of the zeolite per volume of the honeycomb catalyst body]. According to this modification technology, the exhaust gas can come in contact with a large amount of the zeolite at high frequency, and the catalyst function of the zeolite which is dependent on the amount of the zeolite can further effectively be developed.    [Patent Document 1] JP-A-2009-154148    [Patent Document 2] JP-A-2009-255034    [Patent Document 3] JP-A-2005-246314